German Published Patent Application No. 199 58 120 refers to a speed controller that is operable in a so-called ACC mode (adaptive cruise control), and in a stop and go mode.
In the ACC mode, the speed of the vehicle is regulated to a desired speed selected by the driver, provided the roadway ahead of his own vehicle is clear. A distance gap sensor, such as a radar sensor, permits detecting vehicles traveling ahead on his own traffic lane and other obstacles, and adjusting speed, if necessary, in such a way that the immediately preceding vehicle is followed at an appropriate safety distance. The ACC mode is provided, in general, for travel on express highways or well developed highways having flowing traffic, and also for traffic situations characterized by relatively low dynamics and relatively large vehicle separations. Under these conditions, a long-range tracking radar, having comparatively low depth resolution, is sufficient for recording the traffic surroundings. The relative speed of the tracked object is directly measurable with the aid of the Doppler effect. In order to avoid frequent faulty reactions of the system, only moving radar objects are generally considered as relevant target objects, since, in general, it is not to be expected that there are standing objects on the roadway. However, in traffic situations having greater dynamics, such as in slow-moving traffic or stop and go traffic, or even in city traffic, standing targets should also be included in the evaluation. Moreover, in this case, because of the generally shorter vehicle separations, a more detailed detection and evaluation of the traffic situation is also desirable. The ACC mode is unsuitable for these traffic situations and is therefore only able to be activated when the speed of one's own vehicle is above a certain limiting speed, such as above 30 km/h.
Alternatively, the stop and go mode is provided for the lower speed range and affords functions that are not available in the ACC mode, in particular the function of braking one's own vehicle to a standstill, such as when driving upon a traffic jam. Under certain circumstances an automatic restart-up is then also possible, when the preceding vehicle is also set in motion again. These conditions are satisfied, for example, when one's own vehicle has stood still for a relatively short time, and when the target object followed up to the present, that is, the preceding vehicle, has constantly remained in the tracking range of the distance sensor. On the other hand, under other conditions, it may be expedient to deactivate the system altogether, or simply to have it emit a start-up prompt to the driver when the preceding vehicle starts up, and to leave the last decision up to him. For an expanded functionality in the stop and go mode, not only is the detection of standing targets required, but in general an additional close-range sensor system is also desirable, such as in the form of a video system having electronic image evaluation, a close-range radar or a light-optical distance sensor for the close range including the left and right roadway edges, so that suddenly-appearing obstacles may also be detected in time. This more complex detection and evaluation of the traffic environment, which may be required in the stop and go mode, can lead, at high speeds, to faulty reactions or to an overload of the system. For this reason, the stop and go mode is typically only activatable at speeds up to an upper limiting speed, such as up to 40 km/h.
In the overlapping zone between the speed ranges for ACC and Stop and Go modes, that is, in the exemplary systems described, between 30 and 40 km/h, both modes can be activated and the selection of the mode is left to the driver. Special mode selection keys are provided in the known system for selecting the operating mode, using which, the driver is able to activate either the ACC mode or the Stop & Go mode. The active participation of the driver in the selection of the operating mode is regarded as efficacious, because in this way it is made clear to the driver in which mode the system happens to be, and which functions of the speed controller are available. Thus, if the preceding vehicle suddenly stops, and the driver mistakenly assumes that the speed controller is in the Stop & Go mode and relies on the speed controller automatic braking to a standstill may be prevented. However, some drivers feel that the necessity of having to select the operating modes themselves is an impairment of their operating convenience, and that the command keys needed for this purpose make the operating system more involved and complex.